I. Field of the Invention:
This invention relates generally to Fabry-Perot optical bandpass filters, and more particularly to a nonlinear tunable optical bandpass filter.
II. Discussion of the Prior Art:
As those skilled in the art will appreciate, conventional Fabry-Perot filters employ two windows with a high reflective coating on adjacent faces which are separated by a spacer having a distance on the order of the transmittance wavelength. Such filters transmit light waves at a maximum wavelength of twice the spacer optical thickness. Light is also transmitted at shorter wavelengths which are integer fractions of this maximum wavelength (i.e., 1/2, 1/3, 1/4, etc.).
U.S. Pat. No. 4,400,058 discloses a tunable Fabry-Perot filter for transmitting light in the wavelength range from 3-40 microns. In that invention, a pair of low index substrates of refractive index less than 2.4 were positioned between an ajustable spacer means. A non-metallic coating of refractive index of at least 4.0 was deposited on the facing sides of the substrates. Tuning was accomplished by mechanically adjusting the spacer means.
U.S. Pat. No. 4,377,324 discloses another Fabry-Perot type device comprising a pair of low index substrates mounted in parallel relationship so as to present facing sides to each other in an optical path. A coating is placed on the facing sides of each substrate, the coating having an increasing index of refraction away from the sides. Adjustment means are provided to adjust the distance between the two faces, at least from a distance of substantially less than a desired wavelength to at least one-half of the desired wavelength.
U.S. Pat. No. 3,556,640 to Austin discloses a Fabry-Perot type interference filter having a double half-wave configuration. This invention allows precise spectral location of narrow (on the order of 0.1 Angstrom) passbands. Fine adjustment of the transmittance wavelength is obtained using "tuning layers" which are vacuum deposited films fabricated prior to the filter assembly. No spectral tuning or switching is possible with the device.
Other prior art devices have incorporated the properties of conventional Fabry-Perot filters to achieve light modulation, optical switching and narrow passband filters. However, these prior art devices generally are either not solid state devices or are constrained by factors such as transmission of only monochromatic light, use of very thick or very thin active layers, use of polarizers, and precise control of radiation angle of incidence. Prior art devices generally allow only limited tuning over narrow spectral ranges.